Foams of high molecular weight polycarbonates are known (see, for example, DE-AS 1,031,507). Suitable blowing agents are, for example, blowing agents which eliminate CO.sub.2 or inert gases, such as N.sub.2 or CO.sub.2. A distinct reduction in the molecular weights is generally observed. The non-volatile decomposition products of the blowing agent remain in the foams.
It is also known that aromatic polycarbonates can only be satisfactorily processed in completely dry form. Any moisture present leads during processing to a reduction in molecular weight and to the formation of bubbles (cf. Plastics Technology 10, 1964, pages 32 to 36; GB-PS 841,652).
Accordingly, the problem addressed by the present invention was to develop a fine-cell foam which would undergo only a slight reduction, if any, in molecular weight, even during re-extrusion and, optionally, re-foaming.
This problem has been solved by the combination of aluminum hydroxides and acidic additives.